Cockpit visits on AY are not categorically forbidden in my experience and I was actually once "upgraded" to the cockpit when my friend was the cpt and was even allowed to ait through the landing.

That being said, it would be at the discretion of the crew/captain, and service consistensy is not one of AYs strengths. Also, I believe my friend mentioned that cockpit visits are forbidden in some airspaces (unsure whether this applies to JPN flights).

AY flights to Tokyo are on a350 metal, two class cabins with a "economy comfort" section with normal Y seats but some extra legroom. The feeders from LHR are operated by either a350/330 widebodies or narrowbody a321/320/319 No real PY cabin. Cheapest regular fare for return LHR-HEL-NRT is around 500-550 GBP or so. No flights to HND from HEL yet, although that might change soon. There is also a JAL flight HEL-NRT daily.

Transit in HEL has had some inconveniences lately, but mostly in Schengen>Non-Schengen transfers so coming from the UK the experience should be smooth.